1. Field of the Invention
This invention relates generally to the manufacture of optical quality glass. More specifically, it is especially useful for the manufacture of glass sheet made by the overflow downdraw process for the production of TFT/LCD display devices that are widely used for television and computer displays.
2. Description of Related Art
The typical glass manufacturing process includes, in series, a raw material storing, mixing, and feeding system, a glass melting furnace, a molten glass delivery system, a glass forming process, and a finished glass handing system for cutting, cleaning, packaging and shipping.
FIG. 1 shows a typical “Overflow Process” manufacturing system (1). The melting furnace (2) feeds liquid glass (16) of substantially uniform temperature and chemical composition to the finer (3), which removes any gaseous inclusions through the finer vent (15) and feeds a stirring device (4), also known as a stirring apparatus (4). The stirring device (4), including one or more stirrers, thoroughly homogenizes the glass. The stirring device (4) is always placed after the finer (3) in the prior art, to remove inhomogeneities in the glass which may be created in the finer (3).
The glass (16) is then conducted through a cooling and conditioning section (5), into a bowl (6), and down into the downcomer pipe (7), through the joint (14) between the downcomer pipe (7) and the forming apparatus inlet pipe (8), to the inlet of the overflow trough (9). While flowing from the stirring device (4) to the trough (9), the glass (16), especially that which forms the sheet surface, must remain homogeneous. The bowl (6) alters the flow direction from horizontal to vertical and provides a means for stopping the flow of glass (16). A needle (13) is often provided to stop glass flow. The downcomer pipe (7) has two primary functions. It conducts the glass from the bowl (6) to the trough inlet pipe (8) and controls the flow rate of the glass (16) entering the sheet forming apparatus. The downcomer pipe (7) is carefully designed such that, by maintaining it at a specific temperature, the desired glass (16) flow rate is precisely maintained at the desired value. The finer (3), the finer vent (15), the stirring device (4), the cooling and conditioning section (5), the bowl (6), the needle (13), and the downcomer pipe (7) comprise the glass delivery system (10), which conducts and conditions the glass (16) from the furnace to the top of the inlet pipe (8) of the overflow process. The joint (14) between the downcomer pipe (7) and the trough inlet pipe (8) allows for removal of the sheet glass forming apparatus for service as well as providing compensation for the thermal expansion of the process equipment.
The glass (16) flowing from the furnace (2) is at a high temperature (1500° to 1600° C.) and is a Newtonian liquid, but has gaseous inclusion defects and is not a homogeneous mixture. The delivery system (10) delivers the glass to the overflow forming process at the correct temperature (approximately 1225° C.), in a homogeneous state with a minimum of gaseous inclusions or other homogeneity defects.
The molten glass (16) from the delivery system (10), which must be of substantially uniform temperature and chemical composition, enters the sheet forming apparatus through the inlet pipe (8) to the sheet forming trough (9). The glass sheet forming apparatus, which is described in detail in U.S. Pat. Nos. 3,338,696, 6,748,765, and 6,889,526, herein incorporated by reference, is a wedge shaped forming device (9). The glass (16) then flows down each side of the wedge shaped forming device (9), and joins at the pointed bottom edge to form a sheet of molten glass (11). The sheet of molten glass (11) is then cooled to form a solid glass sheet (12) of substantially uniform thickness.
Glass as melted from raw materials has many small bubbles of entrapped gases. These bubbles are considered defects in any glass product which requires optical properties. Bubbles of a size that can be seen by the eye or that interfere with the function of the product must be removed. The process for removing these bubbles is termed either fining or degassing (fining herein). Fining occurs after the glass is melted from raw materials, but before the glass is formed into a finished product. In optical quality glass, this fining process is performed in a “finer” (or refiner), which is constructed of precious metal, typically platinum or a platinum alloy. The fining process is both chemical and physical. Chemicals, termed fining agents, are added to the glass such that the bubbles grow in size as they pass through the glass melting furnace and the finer. Arsenic or Antimony as oxides in the glass are preferred fining agents, but are toxic materials. Tin is another commonly used fining agent, but it is less effective as a fining agent and it chemically reduces platinum, producing tiny particles and causing eventual destruction of the platinum walls. Cerium may also be used as a fining agent, but colors the glass yellow. These are the most used among the fining agents, however, there are others known in the art.
Optical quality glass is unique in that disruptions in the flow path often produce a homogeneity defect. This defect class is called cord and it produces optical distortion in the product. The finer often is designed with baffles as discussed herein. Baffles and the finer vent (15) or vents produce significant flow disruptions. For this reason, in the prior art, the stirring device (4) is placed after the finer (3) in the flow path such that inhomogeneities from the finer are homogenized. Both the finer and stirring device operate at a high temperature of approximately 1600° C. The glass discharged from the stirring device is substantially homogeneous, although the stirring device (4) can itself produce a homogeneity defect, which is discussed herein. To minimize the further creation of inhomogeneities, the cooling and conditioning pipe (5), the bowl (6), and the downcomer pipe (7) are carefully finished (smoothed) on the glass contact surfaces to minimize flow path disruptions. In the delivery system, it is desirable to maintain the flow uniform with no regions of quiescent or recirculating flow and a minimum exposure to the atmosphere. Exposure to the atmosphere can cause volatilization of some of the glass chemicals and thus change the glass composition and properties, potentially introducing homogeneity defects. The temperature of the glass in the delivery system must be maintained above the liquidus temperature of the glass to prevent recrystallization (devitrification) of the glass, which would be an optical defect. The bowl, which in many designs has a free surface, can be a source of cord and devitrification defects.
The fining apparatus is designed such that the removal of the bubbles from the molten glass is optimized. The finer is often very large, resulting in extremely high costs to fabricate because the glass contact surfaces are constructed of platinum or platinum alloy. In the prior art fining process, the bubbles rise to the top of the fining apparatus (finer) where they dissipate to the atmosphere through the finer vent (15). The size of the bubbles that are removed is a function of the size and design of the finer and the viscosity (fluidity) of the molten glass. In the glass industry, these bubbles are called seeds if they are small (less than approximately 1 mm diameter) and blisters if they are large. Seeds are the primary concern as they are small in diameter and therefore are more difficult to remove from the glass.
The glass seed entering the finer at the bottom of the inflow end of the finer must rise to the top of the finer at the outflow end where a vent to the atmosphere is located. The vertical speed of a seed in glass is inversely proportional to the glass viscosity, proportional to the square of the seed diameter, and proportional to the square of the glass density. The glass viscosity is a strong inverse function of temperature, therefore raising the glass temperature to a practical maximum increases the vertical speed of a given size seed. The detection of a seed in an optical product is a strong function of its viewable area, therefore one can use the diameter squared of a seed as the quality criteria. For a given glass, the variation of the glass density in the fining process is a second order effect.
At the very high temperatures, approximately 1600° C., required to substantially reduce the glass viscosity, even the highest quality refractory materials are slowly dissolved by the glass. This introduces contamination and can also generate additional seeds in the glass. In the prior art, a cylindrical platinum or platinum alloy (platinum herein) tube is used for all surfaces (walls) that contact the glass, such that the glass is not contaminated by the dissolution of refractory walls. The cylindrical tube is typically supported externally by refractory material (brick), which has the appropriate strength and insulating properties. The glass in the finer must be maintained at the required elevated temperature. Additionally, the glass entering the inflow end of the finer often must be heated to the desired fining temperature. This is done by either containment of the platinum and refractory finer assembly in a heated (gas or electric) firebox or by electrical heating. The electrical heating of the finer is accomplished by either externally mounted electric windings (normally made of platinum) or the passing of electric current directly through the cylindrical platinum tube, thus using the electrical resistance of the tube to generate the heat.
The prior art design which has been typically used since the start of this practice in the first half of the twentieth century is a cylindrical platinum tube either with or without internal baffles. The primary innovations to date have been in the design of the baffles to alter the flow path and to trap seeds for optimal seed removal. The prior art includes finer designs with and without an internal free surface.
FIG. 2A is a typical baffled finer of the prior art. The molten glass (16) enters the baffled finer (21) at the glass inlet end (23) and flows out the outlet (24). There is a vent (25) at the outlet end (24), which is connected to the atmosphere, to allow the seeds which accumulate at the top of the baffled finer (21) to escape. Some of the baffles (26) have holes (22), which are sized to distribute the flow of the molten glass (16) such that the average residence time for the glass as it flows through the baffled finer (21) is more uniform. Other baffles (28) are designed to move the flow path vertically. There is often a vent (29) in front of a baffle, as baffles also trap the surface seeds into a foam-like accumulation, which breaks down and dissipates into the atmosphere. FIG. 2B shows the movement of seeds (27) through the baffled finer (21). The baffles (26) and (28) make the paths of the seeds (27) in the baffled finer (21) quite tortuous. This allows the smaller seeds greater opportunity to coalesce together and form a larger seed, which in turn will rise faster.
The finer shown in FIGS. 2A and 2B has a diameter of 0.382 meters and a length of 2.5 meters. The glass flow rate is 7.41 metric tons per day. The glass viscosity is 100 poise. The seed diameter is 0.0007 meters. These parameters can be changed by normalizing using the equation:Q1*d12/η1=Q0*d02/η0 
where:
Q equals glass flow,
η equals glass viscosity, and
d equals seed diameter
The prior art stirring device (4) consists of one or more rotating elements. The glass at the tip of the final rotating element is often trapped in a vortex. Glass exiting this vortex has a rotating motion and a different time history than the glass in the main flow path. This can result in a cord homogeneity defect if this glass is part of the salable portion of the product.